Method for fabrication of crack-free ceramic dielectric films

ABSTRACT

The invention provides a process for forming crack-free dielectric films on a substrate. The process comprise the application of a dielectric precursor layer of a thickness from about 0.3 μm to about 1.0 μm to a substrate. The deposition is followed by low temperature heat pretreatment, prepyrolysis, pyrolysis and crystallization step for each layer. The deposition, heat pretreatment, prepyrolysis, pyrolysis and crystallization are repeated until the dielectric film forms an overall thickness of from about 1.5 μm to about 20.0 μm and providing a final crystallization treatment to form a thick dielectric film. Also provided was a thick crack-free dielectric film on a substrate, the dielectric forming a dense thick crack-free dielectric having an overall dielectric thickness of from about 1.5 μm to about 20.0 μm.

CONTRACTUAL ORIGIN OF THE INVENTION

The U.S. Government has rights in this invention pursuant to ContractNo. DE-AC02-06CH11357 between the U.S. Department of Energy and theUniversity of Chicago representing Argonne National Laboratory.

FIELD OF THE INVENTION

The present invention relates to formation of thick ceramic dielectricfilms. More specifically this invention relates to the formation ofthick crack-free ceramic dielectric films, such as lanthanum doped leadzirconate titanate (PLZT) or barium strontium titanate (BST), having anoverall thickness of from 1.5 to about 20 μm.

BACKGROUND OF THE INVENTION

The recent need for passive power electronics with improved performance,high reliability, and reduced size and weight, has driven interest inceramic film on metallic substrates in these applications. The ceramicfilms on metal foils, known as “film-on-foil” technology, in whichceramic films were deposited on base metal foils for embedding into aprinted circuit board. The interest in “film-on-foil” technologyexploits ceramic dielectrics, including the important properties, suchas ferroelectric, piezoelectric, pyroelectric and electro-opticproperties. These properties are utilized in manufacture of nonvolatilesemiconductor memories, thin-film capacitors, pyroelectric infrared (IR)detectors, sensors, surface acoustic wave substrates, opticalwaveguides, and optical memories. Recently, there has been increasedinterest in applying “film-on-foil” to power electronics, such ascapacitors with high capacitance required to work at high voltages.Applying the film-on-foil technology can substantially reduce theproduction cost and improve the volumetric and gravimetric efficienciesof the capacitors.

Important ferroelectric materials for thin-film applications aretypically titanates and niobates with oxygen-octahedral structure types,such as the perovskite structure. Examples of such ferroelectricperovskites include lead titanate (PbTiO₃), lead zirconate (PbZrO₃),lead zirconate titanate [Pb(Zr,Ti)O₃ or PZT], lead lanthanum titanate[(Pb,La)TiO₃], lead lanthanum zirconate [(Pb,La)ZrO₃], lead lanthanumzirconate titanate [(Pb,La)(Zr,Ti)O₃ or PLZT], lead magnesium niobate[Pb(Mg_(1/3)Nb_(2/3))O₃], lead zinc niobate [Pb(Zn_(1/3)Nb_(2/3))O₃],strontium titanate (SrTiO₃), barium titanate (BaTiO₃), barium strontiumtitanate [(Ba,Sr)TiO₃], barium titanate zirconate [Ba(Ti,Zr)O₃],potassium niobate (KNbO₃), potassium tantalate (KTaO₃), and potassiumtantalate niobate [K(Ta,Nb)O₃]. Device applications of ferroelectricthin films require that bulk ferroelectric properties be achieved inthin films. The physical and chemical properties of the film (density,uniformity, stoichiometry, crystal structure, and microstructure) areextremely important. The utilization of ferroelectric thin films forelectronic and optical applications has been hindered by the lack ofproduction processes to form deposits of sufficient thickness.

Particular interest has shown that lead lanthanum zirconate titanate(Pb_(0.92)La_(0.08)Zr_(0.52)Ti_(0.48)O₃, PLZT) films deposited on nickelor copper foils possessed excellent dielectric properties, which arepromising for high power applications such as plug-in hybrid electricvehicles. In power electronics, capacitors with high capacitance arerequired to work at high voltages, typically in the range of 450 to 600V. This requirement imposes an additional challenge to fabricate thickerfilms (>1 μm) that can withstand high voltage. However, in thefabrication process, the deposited films crack easily during heattreatment, due to the well-known critical thickness effect. Due to thiseffect per-layer thickness that can be achieved by conventional sol-gelmethods is generally limited to about 0.2 μm, thus making these methodsunattractive to industry if thicker films are needed.

It has been reported that the critical thickness of lead zirconatetitanate (PZT) films can be substantially increased by introducingpolyvinylpyrrolidone (PVP) into sol-gel solutions (H. Kozuka and S.Takenaka, J. Am. Ceram. Soc. 85 (11) (2002) 2696-2702) and bariumtitanate (H. Kozuka and M. Kajimura, J. Am. Ceram. Soc. 83 (5)(2000)1056-1062). The increased critical thickness of the PZT dielectricis attributed to the structural relaxation effect as PVP suppressed thecondensation reaction because of the strong hydrogen bonds between theamide groups of PVP and the hydroxyl groups of the metalloxane polymers(H. Kozuka and M. Kajimura, J. Am. Ceram. Soc. 83 (5) (2000)1056-1062).However, thick films derived from PVP-containing solutions weregenerally found to be porous due to the thermal decomposition of PVPduring heating (A. Yamano and H. Kozuka, J. Am. Ceram. Soc. 90 (12)(2007)). Pyrolysis temperature had been shown to have a significantimpact on microstructure of the films derived from PVP-modified sol-gelprocess (Z. H. Du, J. Ma, and T. S. Zhang, J. Am. Ceram. Soc. 90 (3)(2007)).

The efficient removal of decomposition byproducts produced by processingaids during dielectric fabrication and the consolidation of the filmraises significant processing issues.

SUMMARY OF INVENTION

An object of the invention is to provide a thick dielectric film havingand a method for forming overcomes many of the disadvantages of theprior art films.

Another object of the present invention is to provide a process for themanufacture of a crack-free ceramic film having an overall/finalthickness from 1.5 μm to about 20.0 μm. In an embodiment the overallthickness is from about 2.0 μm to 10.0 μm. In another embodiment theoverall thickness is from about 2.0 μm to 5.0 μm. A feature of theinvention is to provide a heat treatment process that reduces stressesbrought about differences in thermal expansion in the formation of thickdielectric films. An advantage of the invention is to permit theplacement of multiple layers to form a dielectric with a substantialthickness without formation of thermal stress cracks.

Another object of the present invention is to provide a process for themanufacture dense dielectric ceramic film. A feature of the invention isa process that permits coalescence of a dielectric and the reduction ofvoid space. An advantage of the invention is the formation of a ceramichaving reduced voids thereby forming a continuous dense material.

Another object of the present invention is to provide a process, themethod for forming a dielectric ceramic having a higher dielectricconstant compared to materials fabricated by typically processingmethods. A feature of the invention is to provide a heating process thatefficiently removes most of the processing component that would normallylower the dielectric constant of the final film. An advantage of theinvention is the formation of dense ceramic containing fewer formingaids that lower the dielectric constant of the dielectric film.

In brief, the invention provides a process for forming crack-freedielectric films on a substrate, the process comprising the applicationof a dielectric precursor layer to a substrate, low temperature heatpretreatment, staged prepyrolysis, pyrolysis and crystallization stepfor each layer, repeated until the dielectric film forms a total oroverall thickness of from about 1.5 μm to about 20.0 μm and providing afinal crystallization treatment to form a thick dielectric film. In anembodiment of the invention, the total thickness is from about 2.0 μm toabout 10.0 μm on a substrate (preferably 2.0 μm to about 5.0 μm). Alsoprovided was a thick crack-free dielectric film on a substrate, thedielectric forming a dense thick crack-free dielectric having an overalldielectric thickness of from about 1.5 μm to about 20.0 μm.

The invention provides a process for forming a crack-free dielectric,the process includes providing a substrate and providing a dielectricprecursor soluble gel solution. An initial dielectric precursor sol-gellayer having a thickness from about 0.3 μm to about 1.0 μm is depositedon a substrate. The first dielectric precursor sol-gel layer is heatedat a low temperature preheat from about 100° C. to about 200° C. for aperiod of from about 1 minute to about 30 minutes. In an embodiment ofthe invention temperature preheat from about 100° C. to about 180° C.The temperature is increased to a prepyrolysis (or low temperaturepyrolysis) at a temperature from about 275° C. to about 325° C. in aprepyrolysis step and maintained at the prepyrolysis temperature for aprepyrolysis period of time. In an embodiment, the prepyrolysistemperature is from about 285° C. to about 315° C. The temperature isthen increased to a first pyrolysis temperature from about 375° C. toabout 425° C. and maintained for a first pyrolysis period of time. Thetemperature is then increased to a second pyrolysis temperature fromabout 425° C. to about 475° C. in a second pyrolysis step and maintainedat the temperature at the second pyrolysis temperature for a secondpyrolysis period of time. The temperature is then raised to acrystallization temperature of from about 600° C. to about 800° C. for aperiod of time to crystallize at least one layer. The deposition step,initial heating step, pyrolyzing steps and crystallization step arerepeated to form a dielectric precursor layer having a total thicknessfrom about 1.5 μm to about 20.0 μm on a substrate. In an embodiment ofthe invention, the total thickness of the dielectric film is 2.0 μm toabout 10.0 μm on a substrate. In one embodiment the total thickness fromabout 2.0 μm to about 5.0 μm. In one embodiment of the invention a finaldensification heating step wherein the substrate and the dielectricprecursor are heated to crystallize the dielectric precursor at atemperature from about 600° C. to about 800° C. for a finalcrystallization of time to form crystallized dielectric layer. Inanother embodiment of the invention, a final densification heating stepwherein the substrate and the dielectric precursor are heated tocrystallize the dielectric precursor at a temperature from about 650° C.to about 750° C. for a final crystallization of time to formcrystallized dielectric layer. Preferably, the process forms adielectric film layer having a total thickness from about 1.5 μm toabout 20 μm on a substrate. In one embodiment of the invention theprocess forms a dielectric layer having a total thickness from about 2.0μm to about 10 μm (preferably from about 2.0 μm to about 5.0 μm). Thestep-wise preheat treatment (SPT) produces a dielectric film producingsuperior electrical properties over dielectric films produce by theconventional rapid thermal annealing (RTA) process. In the RTA process,films placed in an alumina boat were directly inserted into a tubefurnace preheated at 450° C. In the SPT process, films were preheated at300° C. for 5 min, then 400° C. for another 5 min, and finally at 450°C. for 10 min (by moving the film into different hot zones withdesignated temperatures in an electric furnace).

In an embodiment of the invention, the low temperature heating time isfrom about 2 minutes to about 10 minutes. In another embodiment of theinvention, the prepyrolysis, first and second pyrolysis times are fromabout 4 to about 10 minutes. Still, in another embodiment of theinvention, the final crystallization time for the process is from about5 to about 40 minutes. In another embodiment of the invention, the finalcrystallization time is from about 10 minutes to about 20 minutes. Theprocess can be used to for a dielectric layer from any dielectric filmforming material; preferably selected from lanthanum doped leadzirconate titanate soluble gel solution (PLZT sol-gel) or a bariumstrontium titanate (BST). The substrate can be any suitable substrate,such as a silicon wafer, a platinized silicon wafer, a base metal(nickel, copper, iron or chromium) or an alloy of a base metal(Hastelloy C or Inconel 625).

BRIEF DESCRIPTION OF DRAWING

The invention together with the above and other objects and advantageswill be best understood from the following detailed description of thepreferred embodiment of the invention shown in the accompanyingdrawings, wherein:

FIG. 1 is an XRD profile of the PLZT films prepared by the SPT and RTAwith different PLZT: PVP ratios.

FIG. 2. SEM images showing the surface and cross-sectional (insets)morphology of the PLZT films: (a) RTA, PLZT:PVP=1:2; (b) SPT,PLZT:PVP=1:2; (c) RTA, PLZT:PVP=1:3; and (d) SPT, PLZT:PVP=1:3.

FIG. 3. Dielectric constant (k) and dissipation factor (D.F.) of thePLZT films as a function of bias field: (a) PLZT:PVP=1:2, 20° C.; (b)PLZT:PVP=1:2, 150° C.; (c) PLZT:PVP=1:3, 20° C.; and (d) PLZT:PVP=1:3,150° C.

FIG. 4. Hysteresis loops of the PLZT films at room temperature: (a)PLZT:PVP=1:2 and (b) PLZT:PVP=1:3.

FIG. 5. Current density relaxation of the PLZT films measured at 100kV/cm at room temperature. Symbols are experimental data and solid linesare the fitting curves.

FIG. 6. Weibull plots and corresponding parameters for dielectricbreakdown strength of the films derived from the PLZT:PVP=1:2 solutionand pyrolyzed by RTA and SPT.

DETAILED DESCRIPTION OF THE INVENTION

The foregoing summary, as well as the following detailed description ofcertain embodiments of the present invention, will be better understoodwhen read in conjunction with the appended drawings.

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralsaid elements or steps, unless such exclusion is explicitly stated.Furthermore, references to “one embodiment” of the present invention arenot intended to be interpreted as excluding the existence of additionalembodiments that also incorporate the recited features. Moreover, unlessexplicitly stated to the contrary, embodiments “comprising” or “having”an element or a plurality of elements having a particular property mayinclude additional such elements not having that property.

Dielectric films are used in electrical components, particularlycapacitors, to inhibit/control the flow of electricity for powermanagement applications. In power electronics applications, such aselectric vehicles, capacitors with high capacitance are required tooperate at voltages in excess of 100 volts (V). In one embodiment of theinvention, capacitors formed from the process of the invention operateat voltages in excess of 50 volts. Typically, this would require amaterial with a dielectric constant (κ)≈1000 at zero bias. Thisrequirement imposes an additional challenge to fabricate thicker filmsin the range of 1.5 μm to 20 μm, while providing a film substantiallyfree from cracks and voids. To achieve this objective the film must forma dense layer substantially free from defects and cracks. The use ofprocessing additives, such as polyvinylpyrrolidone (PVP)—increase theviscosity of the coating solution. The use of the viscosity modifiersraises additional processing issues in the efficient removal ofprocessing aids and modifiers to form a dense dielectric film on asubstrate. To realize this goal the inventors have developed adielectric/support curing process for the fabrication of densecrack-free dielectrics while reducing the overall fabrication time.

The inventors have developed a dielectric fabrication process for theformation of a thick dense dielectric substantially free from voids andcracks that reduce the dielectric constant of the film. Further theinvented process reduces cracking of the dense film therebysignificantly reducing the leakage current density (A/cm²) and improvingthe dielectric breakdown strength of the film. The invented processeffectively fuses/melds multiple layers, thereby consolidating the densefilm into one continuous film, while significantly reducingcracks/fractures.

The invention provides a process for forming crack-free dielectric filmson a substrate. A dielectric precursor layer is applied to a substrate.Initially, the precursor is heated in low temperature heat pretreatmentstep, followed by prepyrolysis, pyrolysis (in two stages) andcrystallization step for each layer. The, steps of deposition, lowtemperature heat pretreatment, staged prepyrolysis, pyrolysis andcrystallization are repeated until the dielectric film forms an overallthickness of from about 1.5 μm to about 20.0 μm and providing a finalcrystallization treatment to form a thick dielectric film. In anembodiment of the invention the total thickness is from about 2.0 μm toabout 10 μm. Also provided was a thick crack-free dielectric film on asubstrate, the dielectric forming a dense thick crack-free dielectrichaving an overall dielectric thickness of from about 1.5 μm to about20.0 μm. Surprisingly the inventors have discovered that the inventedprocess produces a dense film having substantially fewer racks andvoids. The inventors have discovered that the invented process increasesthe dielectric constant of the film by approximately 50% above thedielectric constant produced from the process of the invention. Thissurprising increase in the dielectric constant combined with a reduceddissipation factor (DF) for dielectric films produced by the inventionprovides a dielectric film providing increase performance.

Experimental Procedure

PLZT precursor solutions of 0.6 M concentration were prepared by amodified 2-methoxyethanol synthesis route by using following rawmaterials: 99% lead acetate tri-hydrate, 97% titanium isopropoxide, 70%zirconium n-propoxide in 1-propanol, and 99.9% lanthanum acetate hydrate(all from Sigma-Aldrich Co.). The solution contains 20% excess lead tocompensate for the loss during treatment. To form the chemical solutionfor deposition, PVP (PVP10, Sigma-Aldrich Co., with an average molecularweight of 10,000 g/mol) was added to the PLZT stock solution inPLZT:PVP=1:2 and 1:3 molar ratio (PVP is defined by its monomer). ThePVP-added PLZT solution was aged for approximately 12 h before coating.The aged solution, after passing through a 0.2-μm syringe filter, wasdeposited on a substrate by means of a spin coater (LaurellTechnologies, North Wales, Pa.) at 2000 rpm for 30 sec. The substratewas a platinized silicon wafer with ≈100-nm thick Pt layer (NovaElectronic Materials, Flower Mound, Tex.). After deposition the filmswere first preheated at 100° C. to 200° C. for 1 to 30 min in a furnace.Then, the films were subjected to two different pyrolysis processes:rapid thermal annealing (RTA) and step-wise preheat treatment (SPT). Inthe RTA process, films placed in an alumina boat were directly insertedinto a tube furnace preheated at 450° C. for 10 min. In the SPT process,films were preheated at 300° C. for 5 min, then 400° C. for another 5min, and finally at 450° C. for 10 min (by moving the film intodifferent hot zones with designated temperatures in an electricfurnace). After pyrolysis, all films were crystallized at 650° C. for 10min. The deposition, pyrolysis, and crystallization steps were repeatedto build up a thickness to about 1.6 μm for all samples; in order toavoid the possible thickness effect on dielectric properties. Finalcrystallization and densification were conducted at 650° C. for 30 min.All heat treatments were performed in ambient atmosphere.

Platinum (100-nm thickness) was deposited on samples through a shadowmask by electron-beam evaporation as top electrodes. Samples with Pt topelectrodes were annealed at 450° C. in air for 5 min for electrodeconditioning. A Signatone QuieTemp® probe system with heatable vacuumchuck (Lucas Signatone Corp., Gilroy, Calif.) was used for dielectricproperty characterization. Phase identification was conducted by using aBruker D8 AXS diffractometer with General Area Detector DiffractionSystem. Microstructure observation was performed with a Hitachi S4700field-emission scanning electron microscope (SEM). An Agilent E4980APrecision LCR Meter was used to measure capacitance and dissipationfactor at an applied bias field. Then, dielectric constant wascalculated with the diameter of the electrode (250-μm diameter electrodewas used for all electrical tests) and the thickness of the film.Hysteresis loops were measured by using a Radiant Technologies PrecisionPremier II tester using a field sweeping frequency of 1 kHz. Breakdownstrength and current-voltage characteristics were measured by using aKeithley 237 high-voltage source meter.

FIG. 1 shows XRD patterns of the PLZT films prepared by SPT- andRTA-process with PLZT:PVP ratios of 1:2 and 1:3. For comparison, XRDpattern of a PLZT sample prepared with the same PLZT stock solution butwithout PVP is also included. (111) preferential orientation wasobserved for this sample; however, all the other samples with PVP showeda random orientation, with (110) as the most intense peak. The (111)preferential orientation was attributed to the lattice matching betweenthe (111)-oriented Pt substrate and the PLZT film, while the absence ofthis orientation is likely related to the PVP decomposition. Yamano andKozuka believed that orientation became difficult in PZT films derivedfrom the solution containing PVP due to the large number of nucleationsites provided by the porosity. Except for this difference, all samplesare phase-pure perovskite without any detectable secondary phases, suchas pyrochlore.

Film thickness was determined from the SEM cross-sectional images (FIG.2). The thicknesses of the PLZT films after 5 or 6 deposition cycleswere in the range of 1.5-1.7 μm, corresponding to per-layer thickness of≈266 nm and ≈340 nm for solutions with PLZT:PVP=1:2 and 1:3 ratio,respectively. The SEM cross-sectional image for the RTA process is shownin FIG. 2( a) (1:2 ratio) and 2(c) (1:3 ratio). The SEM cross-sectionalimage for the inventors' SPT process is shown in FIG. 2( b) (1:2 ratio)and 2(d) (1:3 ratio). The film thickness was not influenced by thepyrolysis methods. The relatively low per-layer thickness is due to thelow molecular weight PVP used in the present study, as layer thicknessis mainly determined by viscosity of the solution. The inventors did notobserve the so-called “rosette” structure, which is common inPb-containing ferroelectric films derived from polymer [PVP orpoly(ethylene glycol)]-modified sol-gel solutions (Z. H. Du and T. S.Zhang, and J. Ma, J. Mater. Res. 22 (8) (2007) 2195-2203).

The inventors observed two trends in the examination of the surfacemorphology of the films. First, the number of the pores and their sizeincreased with increasing PVP addition. Second, for samples preparedfrom the same solution (same PVP content), pore sizes were smaller inthe SPT-treated samples. The more PVP added, the more polymer was burnedout eventually; therefore, it is reasonable to expect more residualpores left in the films prepared from the solution with high PVPcontent. The inventors observe that, although the final pyrolysistemperature was the same (450° C.), films treated with additionalthermal heating steps at lower temperatures demonstrated a higher degreeof integrity. Previous research showed that PVP starts to decompose tocarbonaceous species in the temperature range of 250-320° C., and thecarbonaceous species are oxidized at ≈360-460° C. (H. Kozuka and S.Takenaka, J. Am. Ceram. Soc. 85 (11) (2002) 2696-2702). Therefore, wehypothesized that additional heating stages at 300° C. and 400° C. wouldassist PVP to decompose in a gradual manner, preventing it from directlydecomposing into gaseous species in a violent manner, which likelycauses the formation of large pores and even cracks.

FIG. 3 plots dielectric constant and dissipation factor as a function ofapplied bias field for the SPT- or RTA-treated samples with PLZT:PVP=1:2and 1:3; The properties were measured at 20° C. and 150° C. TheSPT-treated samples have much higher dielectric constant than theRTA-treated samples processed under similar conditions. The inventorsobserved the dielectric constant for the SPT-treated vs. the RTA-treatedsamples increases by ≈38% and ≈50% at 20° C. (FIG. 3 a) and 150° C.,(FIG. 3 b) respectively, for the PLZT:PVP=1:2 solution, and it increasesby ≈44% and ≈56% at 20° C. (FIG. 3 c) and 150° C. (FIG. 3 d),respectively, for the PLZT:PVP=1:3 solution. For samples derived fromthe solutions with a given PLZT to PVP ratio, since XRD analysis did notreveal any preferred crystallographic orientation and secondary phase.The inventors believe that the difference in dielectric constant isattributed to the difference in microstructures as a result of thedifferent pyrolysis conditions. Furthermore, dielectric constant forboth the SPT- and the RTA-treated samples drops rapidly with theincrease of the PVP content in the precursor solution. As shown in FIG.2, samples with various PVP contents and pyrolyzed in different waysexhibited different microstructures. In general, SPT-treated sampleswith less PVP content (FIGS. 2 a and 2 b) exhibited a densermicrostructure, and these samples thus showed higher dielectricconstant. The dielectric constant values of the RTA-treated samples withPLZT:PVP=1:2 are close to those of PLZT films deposited by anacetic-acid-based sol-gel process without PVP addition [1]. Dissipationfactor of these samples is about 5-6% at room temperature, and itdecreased slightly to 4-5% at 150° C., which is also at the same levelas our previous results.

FIG. 4 shows polarization-electric field (P-E) hysteresis loops of thePLZT films measured at room temperature. A high electric field up to1000 kV/cm was applied. In general, all films show a relatively slimhysteresis loop, which is desirable for energy storage applications, asthe area enclosed by the charge and discharge curves represents energyloss. Note that with increasing of PVP content, the shape of the loopstarts to become “fatty,” it means that a larger portion of the electricenergy stored would not be retrievable upon discharge. In addition, wefound an increase in average remnant polarization[P_(r)=(+P_(r)−(−P_(r)))/2] and a decrease in average coercive field[E_(c)=(+E_(c)−(−E_(c)))/2] when the pyrolysis process was changed fromRTA to SPT for the films derived from the solution with same PLZT:PVPratio. For the PLZT:PVP=1:2 solution, P_(r) increased from 13.3 to 13.9μC/cm², while for the PLZT:PVP=1:3 solution, P_(r) increased from 12.4to 14.5 μC/cm². In terms of coercive field, E_(c) decreased from 83 to59 kV/cm for the PLZT:PVP=1:2 and from 112 to 91 kV/cm for thePLZT:PVP=1:3 solution. Again, these differences can be explained by themicrostructural features.

Time-relaxation data of leakage current density at 100 kV/cm are givenin FIG. 5. The decay in dielectric relaxation current obeys Curie-vonSchweidler equation (Jonscher, Dielectric Relaxation in Solids, ChelseaDielectrics Press (1983))J=J _(s) +J ₀ t ^(−n)  (1)where J_(s) is the steady-state current density, J₀ is a fittingconstant, t is the relaxation time in second, and n is the slope of thelog-log plot. The calculated steady-state current densities are listedin table 1.

TABLE 1 Steady-state leakage current densities of the PLZT films.Leakage current Samples density (A/cm²) 1:2 RTA 1.38 × 10⁻⁸ 1:2 SPT 5.43× 10⁻⁹ 1:3 RTA 7.20 × 10⁻⁸ 1:3 SPT 4.12 × 10⁻⁸

We can see that it follows a logical trend that films with high PVPcontent and pyrolyzed with the RTA process show higher leakage current.In addition to the microstructural defects that can be used tointeroperate this difference, more residual carbon left inside the filmswith high PVP content and pyrolyzed in a rapid way may also contributeto high leakage current. The leakage current value measured for theSPT-treated sample with PLTZ:PVP=1:2 is close to that of the filmsdeposited on nickel substrates using the same chemical solution butwithout PVP (B. Ma, D. K. Kwon, M. Narayanan, and U. Balachandran,Mater. Lett. 62 (2008) 3573-3575).

Dielectric breakdown strength (BDS) was measured on 25 samples tested ina top-to-bottom electrode configuration. Failure of the sample wasdefined by a 1-μA criterion. The BDS data for the films derived from thePLZT:PVP=1:2 solution presented as Weibull plots (B. Ma, D. K. Kwon, M.Narayanan, and U. Balachandran, Mater. Lett. 62 (2008) 3573-3575), (FIG.6) due to the inherently statistical nature of failure. The samplestreated by the SPT process show slightly higher mean BDS (BDS≈2.1 MV/cm)than the samples treated by the RTA process (BDS≈1.9 MV/cm).Furthermore, their Weibull moduli (β) exhibit a larger difference (β=9.5for the RTA sample and β=27.3 for the SPT sample). Weibull modulus is ameasure of distribution of the data: the higher the value of β, thesmaller the variation of the data. Thus, a higher modulus indicates abetter representation of the sample-to-sample performance as measured bymean breakdown strength. Here, the higher modulus found in theSPT-treated samples is attributed to their denser microstructure (FIG.2), as residual porosity (especially so-called “critical flaws” thatinitiate the breakdown process) is always detrimental to the breakdownstrength (S. Chao, V. Petrovsky, and F. Dogan, J. Mater. Sci. 45 (2010)6685-6693). High breakdown strength, together with small datascattering, is of great importance for ceramic capacitors, asreliability is still one of the biggest concerns for this type ofcapacitor. Here, we demonstrated that with appropriate control of thepyrolysis conditions, PLZT films with high reliability can be fabricatedby a PVP modified sol-gel process.

Generally, the invention provides for a process for forming ahigh-quality ferroelectric PLZT films were prepared by a modifiedsol-gel process. Surprising, the step-wise preheat treatment waseffective to achieve high quality PLZT films, as it reduced the numberand the size of the defects left by the decomposition of sol-gelmodifier (PVP). The PLZT films prepared by the SPT process exhibitedsuperior dielectric properties: dielectric constant ≈860, dissipationfactor ≈0.06, leakage current ≈5.4×10⁻⁹ A/cm², and breakdown strength≈2.1 MV/cm. These values are comparable to those of the films grown bythe sol-gel method without PVP addition. This process is believed to beapplicable for fabrication of film-on-foil capacitors with thickness >1μm and preferably for capacitors with thickness >10 μm, for powerelectronics.

It is to be understood that the above description is intended to beillustrative, and not restrictive. For example, the above-describedembodiments (and/or aspects thereof) may be used in combination witheach other. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the inventionwithout departing from its scope. While the dimensions and types ofmaterials described herein are intended to define the parameters of theinvention, they are by no means limiting, but are instead exemplaryembodiments. Many other embodiments will be apparent to those of skillin the art upon reviewing the above description. The scope of theinvention should, therefore, be determined with reference to theappended claims, along with the full scope of equivalents to which suchclaims are entitled. In the appended claims, the terms “including” and“in which” are used as the plain-English equivalents of the terms“comprising” and “wherein.” Moreover, in the following claims, the terms“first,” “second,” and “third,” are used merely as labels, and are notintended to impose numerical requirements on their objects. Further, thelimitations of the following claims are not written inmeans-plus-function format and are not intended to be interpreted basedon 35 U.S.C. §112, sixth paragraph, unless and until such claimlimitations expressly use the phrase “means for” followed by a statementof function void of further structure.

The present methods can involve any or all of the steps or conditionsdiscussed above in various combinations, as desired. Accordingly, itwill be readily apparent to the skilled artisan that in some of thedisclosed methods certain steps can be deleted or additional stepsperformed without affecting the viability of the methods.

While the invention has been particularly shown and described withreference to a preferred embodiment hereof, it will be understood bythose skilled in the art that several changes in form and detail may bemade without departing from the spirit and scope of the invention.

While the invention has been particularly shown and described withreference to a preferred embodiment hereof, it will be understood bythose skilled in the art that several changes in form and detail may bemade without departing from the spirit and scope of the invention.

The invention claimed is:
 1. A crack-free dielectric comprising: asubstrate; a dielectric layer overlaying the substrate, whereby thelayer is formed by depositing a first dielectric precursor sol-gel layerhaving a thickness from about 0.3 μm to about 1.0 μm on a substrate,heating the first dielectric precursor sol-gel layer at a lowtemperature from about 100° C. to about 200° C. for a low temperatureheating time from about 1 minute to about 30 minutes, increasing thetemperature from about 275° C. to about 325° C. in a prepyrolysis stepand maintaining the temperature at the prepyrolysis temperature for aprepyrolysis period of time, increasing the temperature from about 375°C. to about 425° C. in a first pyrolysis step and maintaining thetemperature at the first pyrolysis temperature for a first pyrolysisperiod of time, increasing the temperature from about 425° C. to about475° C. in a second pyrolysis step and maintaining the temperature atthe second pyrolysis temperature for a second pyrolysis period of time,increasing the temperature from about 600° C. to about 800° C. for aperiod of time to crystallize at least one layer; repeating thedeposition step, initial heating step, pyrolyzing and crystallizationsteps to form a dielectric precursor layer having a total thickness fromabout 1.5 μm to about 20.0 μm on the substrate to form a crystallizeddielectric layer.
 2. The crack-free dielectric of claim 1 furthercomprising heating the substrate and the dielectric precursor tocrystallize the dielectric precursor at a temperature from about 625° C.to about 675° C. for a final crystallization time to form crystallizeddielectric layer.
 3. The crack-free dielectric of claim 1 wherein thedielectric precursor is a titanate selected from the group consisting oflanthanum doped lead zirconate titanate soluble gel solution (PLZTsol-gel) containing polyvinylpyrrolidone and a barium strontium titanate(BST) soluble gel solution sol-gel containing polyvinylpyrrolidone. 4.The crack-free dielectric of claim 1 wherein the low temperature heatingtime is from about 2 minutes to about 10 minutes.
 5. The crack-freedielectric of claim 1 wherein the deposition step, initial heating step,pyrolyzing and crystallization steps are repeated to form a dielectricprecursor layer having a thickness from about 2.0 μm to about 5.0 μm ona substrate.
 6. The crack-free dielectric of claim 2 wherein the finalcrystallization time is from about 10 to about 40 minutes.
 7. Thecrack-free dielectric of claim 1 wherein the substrate is selected fromthe group consisting of silicon, a platinized silicon wafer and a basemetal.